Anthology of Human Repetitive DNA
269
APE
L2
RT
ES
L3
MIR
RT binding site
tRNA-derived
MIR3
RT binding site
tRNA-derived
APE
RT
ORF1
ORF2
ORF
Fig. 7
L2 and L3 autonomous CR1-like retrotransposons and their
nonautonomous companions MIR and MIR3. L2 and L3 encode enzymes
containing endonuclease (APE) and reverse transcriptase (RT) domains. L3-ORF1
is homologous to esterase (ES).
by target site duplications. Furthermore,
instead of the 3
0
polyA tail, L2 and L3
carry the (TGAA)
3
and (GATTCTAT)
2
mi-
crosatellites at their 3
0
termini respectively.
3.1.3.2
Nonautonomous MIR and MIR3
elements
MIR and MIR3 are considered
to be nonautonomous elements ampli±ed
by L2- and L3-encoded proteins, respec-
tively. MIR is a classical 262-bp SINE
element derived from a tRNA-like se-
quence and a 50 bp sequence fragment
homologous to the 3
0
terminus of the L2 el-
ement (Fig. 7). This terminus was probably
recognized by the L2-encoded RT that led
to ampli±cation of MIRs. Approximately
4% of the human genome is made up of
L2 and MIR copies.
MIR3 is viewed as a nonautonomous ele-
ment that parasitized the L3 retroposition
machinery (Fig. 7). Analogously to MIR
and L2, MIR3 and L3 also share the
50-
bp 3
0
termini, which are likely to serve as
binding sites of the L3-encoded RT. In ad-
dition, the
140-bp 5
0
terminal portions of
the MIR3 and MIR consensus sequences
are 81% identical to each other. This por-
tion is homologous to tRNA and probably
includes the pol III internal promoter es-
sential for proliferation of MIR3 and MIR
retroelements. Approximately 0.5% of the
human genome is derived from L3 and
MIR3 elements.
3.2
LTR Retrotransposons
Human long terminal repeat retrotrans-
posons are retrovirus-like elements retro-
transposed in the genome and inherited
by the host from generation to gener-
ation. It is believed that human LTR
retrotransposons are descendants of ex-
ogenous retroviruses that repeatedly in-
fected germlines of human ancestors and
became endogenous. Endogenous retro-
viruses retain their ability to retrotranspose
within cells for some time, but cannot
be transmitted ef±ciently between cells.
Their subsequent evolution reflects adap-
tation to an intracellular environment,
simpli±cation, and formation of nonau-
tonomous copies.
Most human endogenous retroviruses
(HERVs) are extinct and are represented
only by mutated interspersed copies, par-
ticularly by LTRs. The interspersed copies
of LTR retrotransposons constitute around
9% of the human genome. Although LTR
retrotransposons are not as abundant as
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